This invention relates to a modulator and demodulator apparatus as well as a modulation and demodulation method suitable for use with a network.
Modulator-demodulator apparatus (modems) are used widely for transmission of data in analog circuits which use a voice band.
Modems use a main channel for transmitting main data of a terminal or a like apparatus, and a secondary channel for transmitting data for supervision of a network such as signal quality information and a receive level, for example, to a network supervisory apparatus for supervising the network in a centralized condition.
Since a main channel and a secondary channel are provided by frequency division, network supervisory data can be transmitted without influencing main data. Therefore, when a problem occurs, rapid recovery and disconnection of the problem portion can be realized.
Meanwhile, modems employ, as a modulation and demodulation method, frequency shift keying (FSK) for the secondary signal and phase shift keying (PSK), quadrature amplitude modulation (QAM) or some other method for the main signal. Modems employ, as a communication method, a start-stop communication method for the secondary channel and a synchronizing communication method for the main channel.
Since the communication method in each of the main channel and the secondary channel is different, the modulation and demodulation operations of the two channels cannot be performed integrally, but are performed completely separately from each other. Consequently, a large amount of the hardware is required, and the apparatus cannot be reduced in size.
Accordingly, a modulator and demodulator apparatus is needed wherein modulation and demodulation operations for a main channel and a secondary channel are performed integrally so that the hardware construction can be reduced in size without reduction of communication efficiency.
Also a modulator and demodulator apparatus is needed wherein a main channel is divided into a plurality of main channels so that the modulator and demodulator apparatus can modulate and demodulate data of the main channels and a secondary channel obtained by frequency division and can be connected to a plurality of terminals.
Incidentally, modems of the phase shift keying type employ such a technique as described below for a reception section thereof.
In particular, the reception section includes a timing phase discrimination circuit for discriminating the timing phase of a receive signal. The timing phase discrimination circuit receives a vector signal as a timing signal having a real component and an imaginary component after being sampled into a digital value, and discriminates the timing phase of the input vector signal depending upon which one of several divisional discrimination regions of a discrimination plane the phase of the input signal belongs.
Referring to FIG. 19, a block diagram of a phase discrimination circuit is shown. The phase discrimination circuit shown employs, for example, such a discrimination plane 200 as shown in FIG. 17 wherein it is divided into 10 divisional discrimination regions 201 to 210.
In particular, an input vector is rotated by 0.degree., -36.degree., -72.degree., . . . and -324.degree.. Then, the vectors thus rotated are individually moved to the first quadrant. Then, based on the angle of rotation of that vector which belongs to the region (reference discrimination region) 201 after having moved to the first quadrant as described above, the number of the region to which the vector originally belongs is output.
The phase discrimination circuit shown in FIG. 19 has functions which can be represented by a plurality of timing phase discrimination units 320A to 320J provided by a number equal to the number of discrimination regions of the discrimination plane 200 and a discrimination section 325.
The timing phase discrimination units 320B to 320J include rotation sections 321B to 321J for rotating an input vector by respective desired angles, quadrant movement sections 322B to 322J each for moving the thus rotated vector to the first quadrant, angle calculation sections 323B to 323J each for calculating the angle of the vector moved to the first quadrant, and timing phase preceding stage processing sections (timing phase preceding stage processing means) 224B to 224J each for outputting a first value when the vector thus moved does not belong to the reference discrimination region of the discrimination plane and outputting a second value when the vector moved belongs to the reference discrimination region of the discrimination plane.
Further, in the reception section, a result of such timing phase discrimination is supplied to a roll-off filter to optimally correct the phase of the digitally sampled timing signal.
However, to provide a modulator and demodulator apparatus wherein modulation and demodulation operations for a main channel and a secondary channel are integrally performed and another modulator and demodulator apparatus wherein a plurality of main channels are used and data of the main channels and a secondary channel obtained by frequency division can be modulated and demodulated, reduction of memory capacity of the signal reception section of the modulator and demodulator apparatus and improvement of the reception processing algorithm for preventing deterioration of communication efficiency is required.
In particular, in such as phase discrimination circuit as described above, as seen in FIG. 19, to calculate in advance values of an input vector obtained when the input vector is rotated by 0.degree., -36.degree., -72.degree., . . . and -324.degree. and determine whether the vector thus rotated belongs to the reference discrimination region 201 of the discrimination plane 200, values of sin.theta. and cost of the rotational angle .theta. must be stored in memory. This also applied to a result of discrimination with regard to the discrimination region of the discrimination plane 200.
Accordingly, the portion of the memory which can be commonly used is small. Consequently, the memory capacity necessary for realization of an intended result is large, which requires a correspondingly large amount of hardware for the apparatus.
Further, since a roll-off filter for use by a main data reception section of a modem which employs a main channel divided into a plurality of channels must necessarily have a steep filter characteristic, the roll-off (ROF) ratio is set low. However, if phase jumping is performed using a filter having such a low ROF ratio, then the memory for storing coefficients must have a very large capacity. It is difficult to realize such memory capacity.
For example, where a roll-off filter of the double sampling type having 123 taps is employed, the memory capacity of a coefficient ROM (read only memory) necessary for performing phase jumping of 128 phases is given by the following equation (1): EQU 123 taps.times.128/2=7.872 words (1)
Further, although a sample signal is corrected optimally by the roll-off filter part, depending upon the form of a reception (demodulation) signal having a short training pattern such as a signal, for example, of a fast polling modem, data cannot be fetched appropriately, and the communication efficiency may deteriorate.